Efficacy of selected registered fungicides and trifloxystrobin, was evaluated for the control of powdery mildew (Erysiphe pulchra) on container-grown flowering dogwood (Cornus florida). Bare-root trees were transplanted into a pine bark/peat moss substrate and maintained under a 47% shade cloth. All fungicide treatments significantly reduced the incidence of powdery mildew compared with the untreated control. During the trial period, the most consistent control of powdery mildew was given by azoxystrobin [Heritage® 50W] at 0.16 g ai/liter or propiconazole [Banner MAXX®] at 0.07 g ai/liter, respectively. In two of three years, myclobutanil [Eagle® 40W] at 0.24 g ai/liter was equally effective in controlling this disease as the above fungicides. Copper sulfate pentahydrate [Phyton 27®] at 0.74 g ai/liter gave similar control of powdery mildew in three of four years as propiconazole but was not quite as effective in controlling this disease as azoxystrobin. Paraffinic oil at 1% v/v [Sunspray Ultrafine Oil®] was slightly less effective in controlling powdery mildew than azoxystrobin or myclobutanil. However, no more than 6% leaves on the paraffinic oil-treated dogwood were colonized in any year by E. pulchra, compared with 75% colonization for the untreated controls. Although thiophanate methyl [3336™ 4.5F] at 0.84 g ai/liter and triadimefon [Bayleton® WSP] at 0.16 g ai/liter reduced powdery mildew incidence compared with the untreated control, the level of leaf colonization in three of four years was significantly higher than other fungicides. Trifloxystrobin [Compass™ 50W] at the 0.04 g ai/liter rate gave better control of powdery mildew when applied at 1- and 2-week intervals than on a monthly schedule. In contrast, the 0.08 and 0.16 g ai/liter rate of this fungicide were almost equally effective in controlling this disease at 1-, 2-, and 4-week intervals. When applied on the same schedule, myclobutanil (0.24 g ai/liter) gave the same level of powdery mildew control as all three rates of trifloxystrobin. In 1999, moderate to excellent control of powdery mildew with trifloxystrobin, propiconazole, and azoxystrobin resulted in a significant gains in tree dimensions and trunk caliper over those recorded for the untreated flowering dogwood. No symptoms of phytotoxicity were associated with the use of any of the fungicides screened.
2Professor, Department of Entomology and Plant Pathology; e-mail: <firstname.lastname@example.org>.
3Superintendent, Ornamental Horticulture Research Center, Mobile, AL.
4Associate Superintendent, Ornamental Horticulture Research Center, Mobile, AL.
5Former Research Associate, Department of Entomology and Plant Pathology.